Photonic Integrated Circuits (PICs) integrate different optical functionalities on a single photonic chip. The PICs enable the production of complex optical circuits using high volume semiconductor wafer fabrication techniques. Due to the above mentioned abilities, the PICs are utilized in optical communication networks. Accordingly, they offer to reduce component footprint, eliminate multiple packaging issues and multiple optical alignments, and eventually achieve the unprecedented cost efficiency and volume scalability in mass production of consumer photonics products.
In the context of applications, the advantages of PIC become especially compelling when active waveguide devices such as laser, photodetector, and the like, are combined with passive waveguide devices and elements of a waveguide circuit, to form a highly functional photonic system on the chip with minimal optical ports. The active devices that modulate optical signals by electrical means are usually made from artificially grown semiconductors having bandgap structures adjusted to the function and wavelength range of their particular application. These semiconductors are utilized as base material of the PICs. Accordingly, semiconductor based PICs in which several functions such as optical signal detection, modulation, and optical signal emission are implemented in a single monolithic semiconductor chip are a promising solution. Further, indium phosphide (InP) and its related III-V semiconductor material system offer additional benefits as they allow the fabrication of active devices operating in the important wavelength ranges around 1300 nm and 1550 nm, i.e., in the two dominant low-loss transmission windows of the glass fibers. However, even such monolithic integration can provide cost barriers with poor design methodologies, low manufacturing yields, complicated manufacturing processes, and repeated expensive epitaxial growth processes. Accordingly, single step epitaxial wafer growth methodologies in conjunction with established wafer fabrication technologies, have received attention as a means to further enable reduced optical components cost.
Alternatively, gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) may be employed for 850 nm and 1300 nm PICs. Further, PICs may be employed across visible and near ultraviolet regions through exploitation of other tertiary and ternary semiconductor materials employing indium (In), gallium (Ga), aluminum (Al), arsenic (As), and phosphorous (P). The function of any waveguide device within a PIC composed of epitaxially grown semiconductor heterostructures is pre-determined by its band structure and, more particularly by the bandgap wavelength of the waveguide core layer(s), the cladding layer(s), and the substrate. Accordingly, functionally different devices are typically made from different, yet compatible, semiconductor materials although through targeted design some structures can provide optical amplification and photodetection with reversed bias polarity. However, the selection of substrate and waveguide design has a profound impact both on the design and fabrication of the PIC.
In several PICs ranging from wavelength division multiplexers (WDMs), wavelength division demultiplexers (also referred to as WDMs), optical power (channel) monitors, reconfigurable optical add-drop multiplexers (ROADMs), and dynamic gain (channel) equalizers (DGEs/DCEs), at least one multi-wavelength signal is spectrally dispersed, detected, monitored, and processed on a per wavelength basis. For an array of multi-wavelength signals, the array of multi-wavelength signals are monitored and processed on a per wavelength basis and then multiplexed to form a multi-wavelength outgoing signal. These PICs must operate on predetermined channel wavelength plans (i.e., O-band (Original; 1260 nm≦λ≦1360 nm); E-band (Extended; 1360 nm≦λ≦1460 nm); S-band (Short; 1430 nm≦λ≦1530 nm); C-band (Conventional; 1530 nm≦λ≦1565 nm); and L-band (Long; 1565 nm≦λ≦1625 nm),) as the different wavelength optical signals are generally provided from a plurality of remote and discrete transmitters. The channel wavelength plans are defined by the International Telecom Union in ITU-T G.694.1 “Spectral Grids for WDM applications: DWDM Frequency Grid.” Accordingly, the ITU-T G.694.1 defines a fixed grid that exploits channel spacing of 12.5 GHz, 25 GHz, 50 GHz, and 100 GHz according to the equation (1) as shown below:193.1 THz+n*Spacing/1000  (1)where Spacing=12.5 GHz, 25 GHz, 50 GHz, and 100 GHz, and n≧0 within the C and L bands of the optical spectrum.
There is also a flexible grid with channels centered at 193.1 THz+n*0.00625, where n≧0, i.e., at 6.25 GHz centers, and channel bandwidths defined by 12.5 GHz*m, where m≧0. Instead of dense WDM (DWDM) other systems exploit coarse WDM (CWDM) as specified by ITU-T G.694.2 that defines wavelengths from 1271 nm through 1611 nm with a channel spacing of 20 nm.
The temperature stability of the PICs becomes a design constraint over operating temperature ranges of 0° C.≦T≦70° C., i.e., during internal customer premises and telecom installations, and −40° C.≦T≦85° C. for external plant. Further, at channel spacing of ˜0.8 nm (100 GHz) and ˜0.4 nm (50 GHz), the temperature dependencies in terms of nm/° C. become significant. However, InP exhibits a temperature sensitivity of ˜0.1 nm/° C. such that over 0° C.≦T≦70° C. the wavelength will shift ˜7 nm and ˜9/˜18 channels at 100 GHz/50 GHz respectively. As such, temperature control through heaters and thermoelectric coolers has become a standard within InP and other compound semiconductor PICs. However, as we move from considering a single DFB laser through to a 4-channel, 16-channel, and a 40-channel PIC, such as CWDM and DWDM receivers with integrated photodiodes, the die footprint increases significantly, primarily from the WDM component, such that active temperature stabilization becomes increasingly difficult to achieve. Further, there are additional issues that arise with integration, for example, thermal crosstalk between adjacent elements and the like.
Referring now to FIG. 1, a temperature dependent wavelength offsets of InP and SiO2 echelle gratings according to designs of the prior art are shown. First chart 100A shows an expected transmission shift of one channel of an InP Echelle grating WDM with a Gaussian passband characteristic. The peak shifts approximately by +7.6 nm over 85° C. corresponding to dλ/dT≈+0.09 nm/° C. Accordingly, in order to deploy such an InP WDM the effective dn/dTAMB of the WDM must be modified by some form of compensation so that the effect of ambient temperature, TAMB, is reduced. Within the prior art this may be through exploiting a thermoelectric cooler to maintain the InP die temperature at a nominal value, e.g., TInP=35° C. or through the employment of on-chip micro-heaters exploiting resistive metal traces such that the nominal InP die temperature is set above the maximum operating temperature, e.g. TAMB=70° C.-85° C. in order to avoid control issues at TInP=100° C. Within the prior art it is also known that compensating for the inherent refractive index change of a material can be compensated by integrating a second waveguide section with the opposite dn/dT or by modifying the waveguide design to include a cladding material with negative dn/dT such that the effective temperature induced index change of the waveguide is reduced.
However, heaters and thermo-electric coolers can require significant electrical power consumption and also impose complex thermal management requirements upon the die packaging even for just a passive DWDM to ensure uniform temperature even before active devices are considered. Further, negative temperature coefficient materials, i.e., dn/dT<0, are typically polymeric and have low coefficients such that compensating a high dn/dT material such as InP requires significant waveguide real estate to achieve the desired balance. However, other waveguide material systems provide different dn/dT and hence dλ/dT. For example, referring to second chart 100B there is plotted the expected transmission shift of one channel of an SiO2 Echelle grating WDM with a Gaussian passband characteristic. Compared to dnInP/dT≈2×10−4 silica offers dnSiO2/dT≈2×10−5 such that over 85° C. the center wavelength shifts ≈0.8 nm which is equivalent to dλ/dT≈+0.009 nm/° C., an order of magnitude lower than InP.
Furthermore, when an InP waveguide is deposited on an SiO2 substrate, due to mismatch between the lattice structure of the InP waveguide and the SiO2, the InP waveguide cracks due to high stress between the InP waveguide and the SiO2 substrate.
It would therefore be beneficial to provide PIC designers with an alternate WDM compatible with monolithic integration on compound semiconductor PICs that provides for athermal performance such that temperature control of the WDM element can be significantly reduced, thermal management issues are resolved, and that consumes less power.